View Product Detail In Invoice
 

I'd like to show Both the product name and product detail on invoices. I assume it's a matter of adding a field into a template but I don't know which filed or which template. Can anyone point me in the right direction.

I'd like to do the same for viewing order details as well

You mean the product description, the short description, the options...? The file to edit is mail/html/order_invoice.tpl